There are two "possible" time travel scenarios:
- You go back in time and create a new timeline (Back to the future, ...) so you can't go back to your timeline.
- You go back in time and change the unique timeline (EG) so you memories change and whatever you changed it is now reality.
The difference is if the traveler is linked or not to the traveler's timeline. In the first case, the traveler is not linked and can change anything... But in the traveler's timeline nothing has changed and the travel didn't work.
In the second case, the traveler is linked and anything that changes because of the traveler is now that timeline reality and that can create a paradox. Imagine you go back and kill Trump when he was a kid. But you went back because you know the current Trump, the president. But of you kill him while he is a kid he won't become president and you won't know him so you won't travel back to kill him... so he will grow up and become the president and...